TRILL Working Group H. Zhai
Internet-Draft JIT
Intended Status: Standards Track T. Senevirathne
Consultant
R. Perlman
EMC
M. Zhang
Y. Li
Huawei Technologies
Expires: March 28, 2016 September 25, 2015
TRILL: Pseudo-Nickname for Active-Active Accessdraft-ietf-trill-pseudonode-nickname-07.txt
Abstract
The IETF TRILL (TRansparent Interconnection of Lots of Links)
protocol provides support for flow level multi-pathing for both
unicast and multi-destination traffic in networks with arbitrary
topology. Active-active access at the TRILL edge is the extension of
these characteristics to end stations that are multiply connected to
a TRILL campus as discussed in RFC 7379. In this document, the edge
RBridge (TRILL switch) group providing active-active access to such
an end station are represented as a Virtual RBridge. Based on the
concept of Virtual RBridge along with its pseudo-nickname, this
document specifies a method for TRILL active-active access by such
end stations.
Status of This Memo
This Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as
Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/1id-abstracts.htmlH. Zhai, et al [Page 1]

INTERNET DRAFT Pseudo-Nickname September 25, 20151. Introduction
The IETF TRILL protocol [RFC6325] provides optimal pair-wise data
frame forwarding without configuration, safe forwarding even during
periods of temporary loops, and support for multi-pathing of both
unicast and multicast traffic. TRILL accomplishes this by using IS-IS
[IS-IS] [RFC7176] link state routing and encapsulating traffic using
a header that includes a hop count. Devices that implement TRILL are
called RBridges or TRILL switches.
In the base TRILL protocol, an end node can be attached to the TRILL
campus via a point-to-point link or a shared link such as a bridged
LAN (Local Area Network). Although there might be more than one edge
RBridge on a shared link, to avoid potential forwarding loops, one
and only one of the edge RBridges is permitted to provide forwarding
service for end station traffic in each VLAN (Virtual LAN). That
RBridge is referred to as the Appointed Forwarder (AF) for that VLAN
on the link [RFC6325] [RFC6439]. However, in some practical
deployments, to increase the access bandwidth and reliability, an end
station might be multiply connected to several edge RBridges and all
of the uplinks are handled via a Local Active-Active Link Protocol
(LAALP [RFC7379]) such as Multi-Chassis Link Aggregation (MC-LAG) or
Distributed Resilient Network Interconnect (DRNI [802.1AX]). In this
case, it's required that traffic can be ingressed/egressed into/from
the TRILL campus by any of the RBridges for each given VLAN. These
RBridges constitutes an Active-Active Edge (AAE) RBridge group.
With an LAALP, traffic with the same VLAN and source MAC address but
belonging to different flows will frequently be sent to different
member RBridges of the AAE group and then ingressed into TRILL
campus. When an egress RBridge receives such TRILL data packets
ingressed by different RBridges, it learns different VLAN and MAC
address to nickname correspondences continuously when decapsulating
the packets if it has data plane address learning enabled. This issue
is known as the "MAC flip-flopping" issue, which makes most TRILL
switches behave badly and causes the returning traffic to reach the
destination via different paths resulting in persistent re-ordering
of the frames. In addition to this issue, other issues such as
duplicate egressing and loop back of multi-destination frames may
also disturb an end station multiply connected to the member RBridges
of an AAE group [RFC7379].
This document addresses the AAE issues of TRILL by specifying how
members of an edge RBridge group can be represented by a Virtual
RBridge (RBv) and assigned a pseudo-nickname. A member RBridge of
such a group uses a pseudo-nickname, instead of its own nickname, as
the ingress RBridge nickname when ingressing frames received on
attached LAALP links. Other methods are possible: for example the
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INTERNET DRAFT Pseudo-Nickname September 25, 2015
specification in this document and the specification in [MultiAttach]
could be simultaneously deployed for different AAE groups in the same
campus. If the method is [MultiAttach] is used, edge TRILL switches
need to support the capability indicated by the Capability Flags
APPsub-TLV as specified in Section 4.2 of [MultiAttach]. If the
method defined in this document is adopted, all TRILL switches need
to support the Affinity sub-TLV defined in [RFC7176] and [CMT]. For a
TRILL campus that deploys both these AAE methods, TRILL switches are
required to support both methods. However, it is desirable to only
adopt one method in a TRILL campus so that the operating expense,
complexity of troubleshooting, etc, can be reduced.
The main body of this document is organized as follows: Section 2
gives an overview of the TRILL active-active access issues and the
reason that a virtual RBridge (RBv) is used to resolve the issues.
Section 3 gives the concept of a virtual RBridge (RBv) and its
pseudo-nickname. Section 4 describes how edge RBridges can support an
RBv automatically and get a pseudo-nickname for the RBv. Section 5
discusses how to protect multi-destination traffic against disruption
due to Reverse Forwarding Path (RPF) check failure, duplication,
forwarding loops, etc. Section 6 covers the special processing of
native frames and TRILL data packets at member RBridges of an RBv
(also referred to as an Active-Active Edge (AAE) RBridge group).
Section 7 describes the MAC information synchronization among the
member RBridges of an RBv. Section 8 discusses protection against
downlink failure at a member RBridge; and Section 9 gives the
necessary TRILL code points and data structures for a pseudo-nickname
AAE RBridge group.
1.1. Terminology and Acronyms
This document uses the acronyms and terms defined in [RFC6325] and
[RFC7379] and the following additional acronyms:
AAE - Active-active Edge RBridge group, a group of edge RBridges to
which at least one CE is multiply attached with an LAALP. AAE is also
referred to as edge group or Virtual RBridge in this document.
Campus - A TRILL network consisting of TRILL switches, links, and
possibly bridges bounded by end stations and IP routers. For TRILL,
there is no "academic" implication in the name "campus".
CE - Customer Equipment (end station or bridge). The device can be
either physical or virtual equipment.
Data Label - VLAN or FGL.
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DF - Designated Forwarder.
DRNI: Distributed Resilient Network Interconnect. A link aggregation
specified in [802.1AX] that can provide an LAALP between from 1 to 3
CEs and 2 or 3 RBridges.
E-L1FS - Extended Level 1 Flooding Scope [RFC7356].
FGL - Fine-Grained Labeling or Fine-Grained Labeled or Fine-Grained
Label [RFC7172].
LAALP - Local Active-Active Link Protocol [RFC7379] such as MC-LAG or
DRNI.
MC-LAG: Multi-Chassis LAG. Proprietary extensions of Link Aggregation
[802.1AX] that can provide an LAALP between one CE and 2 or more
RBridges.
OE flag - A flag used by the member RBridge of an LAALP to tell other
edge RBridges whether it is willing to share an RBv with other LAALPs
if they multiply attach to the same set of edge RBridges as it. When
this flag for an LAALP is 1, it means that the LAALP needs to be
served by an RBv by itself and is not willing to share, that is, it
should Occupy an RBv Exclusively (OE).
RBv - virtual RBridge, an alias for active-active edge RBridge group
in this document.
vDRB - The Designated RBridge in an RBv. It is responsible for
deciding the pseudo-nickname for the RBv.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
2. Overview
To minimize impact during failures and maximize available access
bandwidth, Customer Equipment (referred to as CE in this document)
may be multiply connected to TRILL campus via multiple edge
RBridges.
Figure 1 shows such a typical deployment scenario, where CE1 attaches
to RB1, RB2, ... RBk and treats all of the uplinks as an LAALP
bundle. Then RB1, RB2, ... RBk constitute an Active-active Edge (AAE)
RBridge group for CE1 in this LAALP. Even if a member RBridge or an
uplink fails, CE1 will still get frame forwarding service from the
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TRILL campus if there are still member RBridges and uplinks available
in the AAE group. Furthermore, CE1 can make flow-based load balancing
across the available member links of the LAALP bundle in the AAE
group when it communicates with other CEs across the TRILL campus
[RFC7379].
----------------------
| |
| TRILL Campus |
| |
----------------------
| | |
+-----+ | +--------+
| | |
+------+ +------+ +------+
|(RB1) | |(RB2) | | (RBk)|
+------+ +------+ +------+
|..| |..| |..|
| +----+ | | | |
| +---|-----|--|----------+ |
| +-|---|-----+ +-----------+ |
| | | +------------------+ | |
LAALP1-->(| | |) (| | |) <--LAALPn
+-------+ . . . +-------+
| CE1 | | CEn |
+-------+ +-------+
Figure 1 Active-Active Connection to TRILL Edge RBridges
By design, an LAALP (say LAALP1) does not forward packets received on
one member port to other member ports. As a result, the TRILL Hello
messages sent by one member RBridge (say RB1) via a port to CE1 will
not be forwarded to other member RBridges by CE1. That is to say,
member RBridges will not see each other's Hellos via the LAALP. So
every member RBridge of LAALP1 thinks of itself as appointed
forwarder for all VLANs enabled on an LAALP1 link and can
ingress/egress frames simultaneously in these VLANs [RFC6439].
The simultaneous flow-based ingressing/egressing can cause some
problems. For example, simultaneous egressing of multi-destination
traffic by multiple member RBridges will result in frame duplication
at CE1 (see Section 3.1 of [RFC7379]); simultaneous ingressing of
frames originated by CE1 for different flows in the same VLAN with
the same source MAC address will result in MAC address flip-flopping
at remote egress RBridges that have data plane address learning
enabled (see Section 3.3 of [RFC7379]). The flip-flopping would in
turn cause packet re-ordering in reverse traffic.
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Edge RBridges learn Data Label and MAC address to nickname
correspondences by default via decapsulating TRILL data packets (see
Section 4.8.1 of [RFC6325] as updated by [RFC7172]). Assuming that
the default data-plane learning is enabled at edge RBridges, MAC
flip-flopping can be solved by using a Virtual RBridge together with
its pseudo-nickname. This document specifies a way to do so.
3. Virtual RBridge and its Pseudo-nickname
A Virtual RBridge (RBv) represents a group of edge RBridges to which
at least one CE is multiply attached using an LAALP. More exactly, it
represents a group of ports on the edge RBridges providing end
station service and the service provided to the CE(s) on these ports,
through which the CE(s) are multiply attached to the TRILL campus
using LAALP(s). Such end station service ports are called RBv ports;
in contrast, other access ports at edge RBridges are called regular
access ports in this document. RBv ports are always LAALP connecting
ports, but not vice versa (see Section 4.1). For an edge RBridge, if
one or more of its end station service ports are ports of an RBv,
that RBridge is a member RBridge of that RBv.
For the convenience of description, a Virtual RBridge is also
referred to as an Active-Active Edge (AAE) group in this document. In
the TRILL campus, an RBv is identified by its pseudo-nickname, which
is different from any RBridge's regular nickname(s). An RBv has one
and only one pseudo-nickname. Each member RBridge (say RB1, RB2 ...,
RBk) of an RBv (say RBvn) advertises RBvn's pseudo-nickname using a
Nickname sub-TLV in its TRILL IS-IS LSP (Link State PDU) [RFC7176]
and SHOULD do so with maximum priority of use (0xFF), along with
their regular nickname(s). (Maximum priority is recommended to avoid
the disruption to an AAE group that would occur if the nickname were
taken away by a higher priority RBridge.) Then, from these LSPs,
other RBridges outside the AAE group know that RBvn is reachable
through RB1 to RBk.
A member RBridge (say RBi) loses its membership in RBvn when its last
port in RBvn becomes unavailable due to failure, re-configuration,
etc. Then RBi removes RBvn's pseudo-nickname from its LSP and
distributes the updated LSP as usual. From those updated LSPs, other
RBridges know that there is no path to RBvn through RBi now.
When member RBridges receive native frames on their RBv ports and
decide to ingress the frames into the TRILL campus, they use that
RBv's pseudo-nickname instead of their own regular nicknames as the
ingress nickname to encapsulate them into TRILL Data packets. So when
these packets arrive at an egress RBridge, even if they are
originated by the same end station in the same VLAN but ingressed by
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different member RBridges, no address flip-flopping is observed on
the egress RBridge when decapsulating these packets. (When a member
RBridge of an AAE group ingresses a frame from a non-RBv port, it
still uses its own regular nickname as the ingress nickname.)
Since RBv is not a physical node and no TRILL frames are forwarded
between its ports via an LAALP, pseudo-node LSP(s) MUST NOT be
created for an RBv. RBv cannot act as a root when constructing
distribution trees for multi-destination traffic and its pseudo-
nickname is ignored when determining the distribution tree root for
TRILL campus [CMT]. So the tree root priority of RBv's nickname MUST
be set to 0, and this nickname MUST NOT be listed in the "s"
nicknames (see Section 4.5 of [RFC6325]) by the RBridge holding the
highest priority tree root nickname.
NOTE: In order to reduce the consumption of nicknames, especially in
large TRILL campus with lots of RBridges and/or active-active
accesses, when multiple CEs attach to the exact same set of edge
RBridges via LAALPs, those edge RBridges should be considered as a
single RBv with a single pseudo-nickname.
4. Member RBridges Auto-Discovery
Edge RBridges connected to a CE via an LAALP can automatically
discover each other with minimal configuration through exchange of
LAALP connection information.
From the perspective of edge RBridges, a CE that connects to edge
RBridges via an LAALP can be identified by the ID of the LAALP that
is unique across the TRILL campus (for example, the MC-LAG or DRNI
System ID [802.1AX]), which is referred to as an LAALP ID in this
document. On each of such edge RBridges, the access port to such a CE
is associated with an LAALP ID for the CE. An LAALP is considered
valid on an edge RBridge only if the RBridge still has an operational
downlink to that LAALP. For such an edge RBridge, it advertises a
list of LAALP IDs for its valid local LAALPs to other edge RBridges
via its E-L1FS FS-LSP(s) [RFC7356][rfc7180bis]. Based on the LAALP
IDs advertised by other RBridges, each RBridge can know which edge
RBridges could constitute an AAE group (See Section 4.1 for more
details). Then one RBridge is elected from the group to allocate an
available nickname (the pseudo-nickname) for the group (See Section4.2 for more details).
4.1. Discovering Member RBridge for an RBv
Take Figure 2 as an example, where CE1 and CE2 multiply attach to
RB1, RB2 and RB3 via LAALP1 and LAALP2 respectively; CE3 and CE4
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RBv with other LAALPs if they multiply attach to exact the same set
of edge RBridges as it. For an LAALP (for example LAALP3), if its OE-
flag is one, it means that LAALP3 does not want to share, so it MUST
Occupy an RBv Exclusively (OE). Support of OE is optional. RBridges
that do not support OE ignore the OE bit and act as if it was zero
(see Section 11 on Configuration Consistency).
Otherwise, the LAALP (for example LAALP1) will share an RBv with
other LAALPs if possible. By default, this flag is set to zero. For
an LAALP, this flag is considered 1 if any edge RBridge advertises it
as one (see Section 9.1).
In the above table, there might be some LAALPs that attach to a
single RBridge due to mis-configuration or link failure, etc. Those
LAALPs are considered as invalid entries. Then each of the LAALP
related edge RBridges performs the following algorithm to decide
which valid LAALPs can be served by an RBv.
Step 1: Take all the valid LAALPs that have their OE-flags set to
1 out of the table and create an RBv per such LAALP.
Step 2: Sort the valid LAALPs left in the table in descending
order based on the number of RBridges in their associated set of
multi-homed RBridges. In the case that several LAALPs have same
number of RBridges, these LAALPs are then ordered in ascending
order in the proper places of the table based on their LAALP IDs
considered as unsigned integers. (for example, in the above table,
both LAALP1 and LAALP2 have 3 member RBridges, assuming LAALP1 ID
is smaller than LAALP2 ID, so LAALP1 is followed by LAALP2 in the
ordered table.)
Step 3: Take the first valid LAALP (say LAALP_i) with the maximum
set of RBridges, say S_i, out of the table and create a new RBv
(Say RBv_i) for it.
Step 4: Walk through the remaining valid LAALPs in the table one
by one, pick up all the valid LAALPs that have their sets of
multi-homed RBridges contain exactly the same RBridges as that of
LAALP_i and take them out of the table. Then appoint RBv_i as the
servicing RBv for those LAALPs.
Step 5: Repeat Step 3-4 for any LAALPs left until all the valid
entries in the table are associated with an RBv.
After performing the above steps, all the 4 RBridges know that LAALP3
is served by an RBv, say RBv1, which has RB3 and RB4 as member
RBridges; LAALP1 and LAALP2 are served by another RBv, say RBv2,
which has RB1, RB2 and RB3 as member RBridges; and LAALP4 is served
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by RBv3, which has RB3 and RB4 as member RBridges, shown as follows:
RBv Serving LAALPs Member RBridges
----- ------------------- ---------------
RBv1 {LAALP3} {RB3, RB4}
RBv2 {LAALP1, LAALP2} {RB1, RB2, RB3}
RBv3 {LAALP4} {RB3, RB4}
In each RBv, one of the member RBridges is elected as the vDRB
(Designated RBridge) of the RBv. Then this RBridge picks up an
available nickname as the pseudo-nickname for the RBv and announces
it to all other member RBridges of the RBv via its TRILL E-L1FS LSPs
(refer to Section 9.2 for the relative extended sub-TLVs).
4.2. Selection of Pseudo-nickname for RBv
As described in Section 3, in the TRILL campus, an RBv is identified
by its pseudo-nickname. In an AAE group, one member RBridge is
elected for the duty to select a pseudo-nickname for this RBv; this
RBridge is called Designated RBridge of the RBv (vDRB) in this
document. The winner is the RBridge with the largest IS-IS System ID
considered as an unsigned integer, in the group. Then based on its
TRILL IS-IS link state database and the potential pseudo-nickname(s)
reported in the PN-LAALP-Membership sub-TLVs by other member RBridges
of this RBv (see Section 9.1 for more details), the vDRB selects an
available nickname as the pseudo-nickname for this RBv and advertises
it to the other RBridges via its E-L1FS FS-LSP(s) (see Section 9.2
and [rfc7180bis]). Except as provided below, the selection of a
nickname to use as the pseudo-nickname follows the usual TRILL rules
given in [RFC6325] as updated by [rfc7180bis].
To reduce the traffic disruption caused by nickname changing, if
possible, vDRB SHOULD attempt to reuse the pseudo-nickname recently
used by the group when selecting nickname for the RBv. To help the
vDRB to do so, each LAALP related RBridge advertises a re-using
pseudo-nickname for each of its LAALPs in its LAALP Membership sub-
TLV if it has used such a pseudo-nickname for that LAALP recently.
Although it is up to the implementation of the vDRB as to how to
treat the re-using pseudo-nicknames, the following is RECOMMENDED:
o If there are multiple available re-using pseudo-nicknames that are
reported by all the member RBridges of some LAALPs in this RBv,
the available one that is reported by the largest number of such
LAALPs is chosen as the pseudo-nickname for this RBv. If a tie
exists, the re-using pseudo-nickname with the smallest value
considered as an unsigned integer is chosen.
o If only one re-using pseudo-nickname is reported, it SHOULD be
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chosen if available.
If there is no available re-using pseudo-nickname reported, the vDRB
selects a nickname by its usual method.
Then the selected pseudo-nickname is announced by the vDRB to other
member RBridges of this RBv in the PN-RBv sub-TLV (see Section 9.2).
5. Distribution Trees and Designated Forwarder
In an AAE group, as each of the member RBridges thinks it is the
appointed forwarder for VLAN x, without changes made for active-
active connection support, they would all ingress/egress frames
into/from TRILL campus for all VLANs. For multi-destination frames,
more than one member RBridges ingressing them may cause some of the
resulting TRILL Data packets to be discarded due to failure of
Reverse Path Forwarding (RPF) Check on other RBridges; for a multi-
destination traffic, more than one RBridges egressing it may cause
local CE(s) receiving duplication frame. Furthermore, in an AAE
group, a multi-destination frame sent by a CE (say CEi) may be
ingressed into TRILL campus by one member RBridge, then another
member RBridge will receive it from TRILL campus and egress it to
CEi, which will result in loop back of frame for CEi. These problems
are all described in [RFC7379].
In the following sub-sections, the first two issues are discussed in
Section 5.1 and Section 5.2, respectively; the third one is discussed
in Section 5.3.
5.1. Different Trees for Different Member RBridges
In TRILL, RBridges normally use distribution trees to forward multi-
destination frames. (Under some circumstances they can be unicast as
specified in [RFC7172].) An RPF Check along with other checking is
used to avoid temporary multicast loops during topology changes
(Section 4.5.2 of [RFC6325]). The RPF Check mechanism only accepts a
multi-destination frame ingressed by an RBridge RBi and forwarded on
a distribution tree if it arrives at another RBridge RBn on the
expected port. If arriving on any other port, the frame MUST be
dropped.
To avoid address flip-flopping on remote RBridges, member RBridges
use RBv's pseudo-nickname instead of their regular nicknames as
ingress nickname to ingress native frames, including multi-
destination frames. From the view of other RBridges, these frames
appear as if they were ingressed by the RBv. When multi-destination
frames of different flows are ingressed by different member RBridges
of an RBv and forwarded along the same distribution tree, they may
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arrive at RBn on different ports. Some of them will violate the RPF
Check principle at RBn and be dropped, which will result in lost
traffic.
In an RBv, if different member RBridge uses different distribution
trees to ingress multi-destination frames, the RPF Check violation
issue can be fixed. Coordinated Multicast Trees (CMT) [CMT] proposes
such an approach, and makes use of the Affinity sub-TLV defined in
[RFC7176] to tell other RBridges which trees a member RBridge (say
RBi) may choose when ingressing multi-destination frames; then all
RBridges in the TRILL campus can calculate RPF Check information for
RBi on those trees taking the tree affinity information into account
[CMT].
This document uses the approach proposed in [CMT] to fix the RPF
Check violation issue. Please refer to [CMT] for more details of the
approach.
5.2. Designated Forwarder for Member RBridges
Take Figure 3 as an example, where CE1 and CE2 are served by an RBv
that has RB1 and RB2 as member RBridges. In VLAN x, the three CEs can
communicate with each other.
---------------------
/ \ +-----+
| TRILL Campus |---| RBn |
\ / +-----+
-----------------------
| |
+----+ +------+
| |
+---------+ +--------+
| RB1 | | RB2 |
| oooooooo|oooooooooooooooo|ooooo |
+o--------+ RBv +-----o--+
o|oooo|oooooooooooooooooooo|o|o |
| +--|--------------------+ | |
| | +---------+ +----------+ |
(| |)<-LAALP1 (| |)<-LAALP2 |
+-------+ +-------+ +-------+
| CE1 | | CE2 | | CE3 |
+-------+ +-------+ +-------+
Figure 3 A Topology with Multi-homed and Single-homed CEs
When a remote RBridge (say RBn) sends a multi-destination TRILL Data
packet in VLAN x (or the FGL that VLAN x maps to if the packet is
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FGL), both RB1 and RB2 will receive it. As each of them thinks it is
the appointed forwarder for VLAN x, without changes made for active-
active connection support, they would both forward the frame to
CE1/CE2. As a result, CE1/CE2 would receive duplicate copies of the
frame through this RBv.
In another case, assume CE3 is single-homed to RB2. When it transmits
a native multi-destination frame onto link CE3-RB2 in VLAN x, the
frame can be locally replicated to the ports to CE1/CE2, and also
encapsulated into TRILL Data packet and ingressed into TRILL campus.
When the packet arrives at RB1 across the TRILL campus, it will be
egressed to CE1/CE2 by RB1. Then CE1/CE2 receives duplicate copies
from RB1 and RB2.
In this document, the Designated Forwarder (DF) for a VLAN is
introduced to avoid the duplicate copies. The basic idea of the DF is
to elect one RBridge per VLAN from an RBv to egress multi-destination
TRILL Data traffic and replicate locally-received multi-destination
native frames to the CEs served by the RBv.
Note that the DF has an effect only on the egressing/replicating of
multi-destination traffic. It has no effect on the ingressing,
forwarding, or egressing of unicast frames. Furthermore, the DF check
is performed only for RBv ports, not on regular access ports.
Each RBridge in an RBv elects a DF using the same algorithm which
guarantees the same RBridge elected as DF per VLAN by all members of
the RBv.
Assuming there are m LAALPs and k member RBridges in an RBv; each
LAALP is referred to as LAALPi where 0 <= i < m, and each RBridge is
referred to as RBj where 0 <= j < k, the DF election algorithm per
VLAN is as follows:
Step 1: For LAALPi, sort all the RBridges in numerically ascending
order based on SHA-256(System IDj | LAALP IDi) considered as an
unsigned intger, where SHA-256 is the hash function in [RFC6234],
"System IDj" is the 6-byte IS-IS System ID of RBj, "|" means
concatenation, and LAALP IDi is the LAALP ID for LAALPi. System ID
and LAALP ID are considered as byte strings. In the case of a tie,
the tied RBridges are sorted in numerically ascending order by
their System IDs considered as unsigned integers.
Step 2: Each RBridge in the numerically sorted list is assigned a
monotonically increasing number j, such that increasing number j
corresponds to its position in the sorted list, i.e., the first
RBridge (the one with the smallest SHA-256(System ID | LAALP ID))
is assigned zero and the last is assigned k-1.
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Step 3: For each VLAN ID n, choose the RBridge whose number equals
(n mod k) as the DF.
Step 4: Repeat Step 1-3 for the remaining LAALPs until there is a
DF per VLAN per LAALP in the RBv.
For a multi-destination native frame of VLAN x received, if RBi is an
LAALP attached RBridge, there are three cases where RBi replicates
the multi-destination frame, as follows:
1) Local replication of the frame to regular (non-AAE) access
ports as per [RFC6325] (and [RFC7172] for FGL).
2) RBv ports associated with the same pseudo-nickname as that of
the incoming port, no matter whether RBi is the DF for the
frame's VLAN on the outgoing ports except that the frame MUST
NOT be replicated back to the incoming port. RBi cannot simply
depend on the DF to forward the multi-destination frame back
into the AAEs associated with pseudo-nickname as that would
cause the source CE to get the frame back, which is a violation
of basic Ethernet properties. The DF will not forward such a
frame back into the AAE due to ingress nickname filtering as
described in Section 5.3.
3) RBv ports on which RBi is the DF for the frame's VLAN while
they are associated with different pseudo-nickname(s) to that
of the incoming port.
For a multi-destination TRILL Data packet received, RBi MUST NOT
egress it out of the RBv ports where it is not DF for the frame's
Inner.VLAN (or for the VLAN corresponding to the Inner.Label if the
packet is an FGL one). Otherwise, whether or not egressing it out of
such ports is further subject to the filtering check result of the
frame's ingress nickname on these ports (see Section 5.3).
5.3. Ingress Nickname Filtering
As shown in Figure 3, CE1 may send multi-destination traffic in VLAN
x to TRILL campus via a member RBridge (say RB1). The traffic is then
TRILL-encapsulated by RB1 and delivered through the TRILL campus to
multi-destination receivers. RB2 may receive the traffic, and egress
it back to CE1 if it is the DF for VLAN x on the port to LAALP1. Then
the traffic loops back to CE1 (see Section 3.2 of [RFC7379).
To fix the above issue, an ingress nickname filtering check is
required by this document. The idea is to check the ingress nickname
of a multi-destination TRILL Data packet before egressing a copy of
it out of an RBv port. If the ingress nickname matches the pseudo-
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nickname of the RBv (associated with the port), the filtering check
should fail and the copy MUST NOT be egressed out of that RBv port.
Otherwise, the copy is egressed out of that port if it has also
passed other checks, such as the appointed forwarder check in Section4.6.2.5 of [RFC6325] and the DF check in Section 5.2.
Note that this ingress nickname filtering check has no effect on the
multi-destination native frames received on access ports and
replicated to other local ports (including RBv ports), since there is
no ingress nickname associated with such frames. Furthermore, for the
RBridge regular access ports, there is no pseudo-nickname associated
with them; so no ingress nickname filtering check is required on
those ports.
More details of data packet processing on RBv ports are given in the
next section.
6. TRILL Traffic Processing
This section provides more details of native frame and TRILL Data
packet processing as it relates to the RBv's pseudo-nickname.
6.1. Native Frames Ingressing
When RB1 receives a unicast native frame from one of its ports that
has end-station service enabled, it processes the frame as described
in Section 4.6.1.1 of [RFC6325] with the following exception.
o If the port is an RBv port, RB1 uses the RBv's pseudo-nickname,
instead of one of its regular nickname(s) as the ingress nickname
when doing TRILL encapsulation on the frame.
When RB1 receives a native multi-destination (Broadcast, Unknown
unicast or Multicast) frame from one of its access ports (including
regular access ports and RBv ports), it processes the frame as
described in Section 4.6.1.2 of [RFC6325] with the following
exceptions.
o If the incoming port is an RBv port, RB1 uses the RBv's pseudo-
nickname, instead of one of its regular nickname(s) as the ingress
nickname when doing TRILL encapsulation on the frame.
o For the copies of the frame replicated locally to RBv ports, there
are two cases as follows:
- If the outgoing port(s) is associated with the same pseudo-
nickname as that of the incoming port but not with the same
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LAALP as the incoming port, the copies are forwarded out of
that outgoing port(s) after passing the appointed forwarder
check for the frame's VLAN. That is to say, the copies are
processed on such port(s) as Section 4.6.1.2 of [RFC6325].
- Else, the Designated Forwarder (DF) check is also made on the
outgoing ports for the frame's VLAN after the appointed
forwarder check. The copies are not output through the ports
that failed the DF check (i.e., RB1 is not DF for the frame's
VLAN on the ports); otherwise, the copies are forwarded out of
the ports that pass the DF check (see Section 5.2).
For such a frame received, the MAC address information learned by
observing it, together with the LAALP ID of the incoming port SHOULD
be shared with other member RBridges in the group (see Section 7).
6.2. Egressing TRILL Data Packets
This section describes egress processing of the TRILL Data packets
received on an RBv member RBridge (say RBn). Section 6.2.1 describes
the egress processing of unicast TRILL Data packets and Section 6.2.2
specifies the multi-destination TRILL Data packets egressing.
6.2.1. Unicast TRILL Data Packets
When receiving a unicast TRILL data packet, RBn checks the egress
nickname in the TRILL header of the packet. If the egress nickname
is one of RBn's regular nicknames, the packet is processed as defined
in Section 4.6.2.4 of [RFC6325].
If the egress nickname is the pseudo-nickname of a local RBv, RBn is
responsible for learning the source MAC address, unless data plane
learning has been disabled. The learned {Inner.MacSA, Data Label,
ingress nickname} triplet SHOULD be shared within the AAE group as
described in Section 7.
Then the packet is de-capsulated to its native form. The Inner.MacDA
and Data Label are looked up in RBn's local forwarding tables, and
one of the three following cases will occur. RBn uses the first case
that applies and ignores the remaining cases:
o If the destination end station identified by the Inner.MacDA and
Data Label is on a local link, the native frame is sent onto that
link with the VLAN from the Inner.VLAN or VLAN corresponding to
the Inner.Label if the packet is FGL.
o Else if RBn can reach the destination through another member
RBridge RBk, it tunnels the native frame to RBk by re-
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encapsulating it into a unicast TRILL Data packet and sends it to
RBk. RBn uses RBk's regular nickname, instead of the pseudo-
nickname as the egress nickname for the re-encapsulation, and the
ingress nickname remains unchanged (somewhat similar to Section2.4.2.1 of [rfc7180bis]). If the hop count value of the packet is
too small for it to reach RBk safely, RBn SHOULD increase that
value properly in doing the re-encapsulation. (NOTE: When
receiving that re-encapsulated TRILL Data packet, as the egress
nickname of the packet is RBk's regular nickname rather than the
pseudo-nickname of a local RBv, RBk will process it as Section4.6.2.4 of [RFC6325], and will not re-forward it to another
RBridge.)
o Else, RBn does not know how to reach the destination; it sends the
native frame out of all the local ports on which it is appointed
forwarder for the Inner.VLAN (or appointed forwarder for the VLAN
into which the Inner.Label maps on that port for FGL TRILL Data
packet [RFC7172]).
6.2.2. Multi-Destination TRILL Data Packets
When RB1 receives a multi-destination TRILL Data Packet, it checks
and processes the packet as described in Section 4.6.2.5 of [RFC6325]
with the following exception.
o On each RBv port where RBn is the appointed forwarder for the
packet's Inner.VLAN (or for the VLAN to which the packet's
Inner.Label maps on that port if it is an FGL TRILL Data packet),
the Designated Forwarder check (see Section 5.2) and the Ingress
Nickname Filtering check (see Section 5.3) are further performed.
For such an RBv port, if either the DF check or the filtering
check fails, the frame MUST NOT be egressed out of that port.
Otherwise, it can be egressed out of that port.
7. MAC Information Synchronization in Edge Group
An edge RBridge, say RB1 in LAALP1, may have learned a { MAC address,
Data Label } to nickname correspondence for a remote host h1 when h1
sends a packet to CE1. The returning traffic from CE1 may go to
another member RBridge of LAALP1, for example RB2. RB2 may not have
that correspondence stored. Therefore it has to do the flooding for
unknown unicast. Such flooding is unnecessary since the returning
traffic is almost always expected and RB1 had learned the address
correspondence. To avoid the unnecessary flooding, RB1 SHOULD share
the correspondence with other RBridges of LAALP1. RB1 synchronizes
the correspondence by using the MAC-RI sub-TLV [RFC6165] in its
ESADI-LSPs [RFC7357].
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On the other hand, RB2 has learned the MAC address and Data Label of
CE1 when CE1 sends a frame to h1 through RB2. The returning traffic
from h1 may go to RB1. RB1 may not have CE1's MAC address and Data
Label stored even though it is in the same LAALP for CE1 as RB2.
Therefore it has to flood the traffic out of all its access ports
where it is appointed forwarder for the VLAN (see Section 6.2.1) or
the VLAN the FGL maps to on that port if the packet is FGL. Such
flooding is unnecessary since the returning traffic is almost always
expected and RB2 had learned the CE1's MAC and Data Label
information. To avoid that unnecessary flooding, RB2 SHOULD share the
MAC address and Data Label with other RBridges of LAALP1. RB2
synchronizes the MAC address and Data Label by enclosing the relative
MAC-RI TLV within a pair of boundary TRILL APPsub-TLVs for LAALP1
(see Section 9.3) in its ESADI-LSP [RFC7357]. After receiving the
enclosed MAC-RI TLVs, the member RBridges of LAALP1 (i.e., LAALP1
related RBridges) treat the MAC address and Data Label as if it was
learned by them locally on their member port of LAALP1; the LAALP1
unrelated RBridges just ignore LAALP1's boundary APPsub-TLVs and
treat the MAC address and Data Label as specified in [RFC7357].
Furthermore, in order to make the LAALP1 unrelated RBridges know that
the MAC and Data Label is reachable through the RBv that provides
service to LAALP1, the Topology-id/Nickname field of the MAC-RI TLV
SHOULD carry the pseudo-nickname of the RBv rather than zero or one
of the originating RBridge's (i.e., RB2's) regular nicknames.
8. Member Link Failure in RBv
As shown in Figure 4, suppose the link RB1-CE1 fails. Although a new
RBv will be formed by RB2 and RB3 to provide active-active service
for LAALP1 (see Section 5), the unicast traffic to CE1 might still be
forwarded to RB1 before the remote RBridge learns CE1 is attached to
the new RBv. That traffic might be disrupted by the link failure.
Section 8.1 discusses the failure protection in this scenario.
However, for multi-destination TRILL Data packets, they can reach all
member RBridges of the new RBv and be egressed to CE1 by either RB2
or RB3 (i.e., the new DF for the traffic's Inner.VLAN or the VLAN the
packet's Inner.Label maps to in the new RBv). Although there might be
a transient hang time between failure and the establishment of the
new RBv, special actions to protect against downlink failure for such
multi-destination packets is not needed.
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the LSPs advertised by different member RBridges of that LAALP. In
that case, the flag for that LAALP is considered as 1.
o RESV (7 bits): MUST be transmitted as zero and ignored on receipt.
o Size (1 byte): Size of remaining part of LAALP RECORD (2 plus
length of the LAALP ID).
o Re-using Pseudo-nickname (2 bytes): Suggested pseudo-nickname of
the AAE group serving the LAALP. If the LAALP is not served by any
AAE group, this field MUST be set to zero. It is used by the
originating RBridge to help the vDRB to reuse the previous pseudo-
nickname of an AAE group (see Section 4.2).
o LAALP ID (variable): The ID of the LAALP. See Section 9.4.
On receipt of such an APPsub-TLV, if RBn is not an LAALP related edge
RBridge, it ignores the sub-TLV; otherwise, it parses the sub-TLV.
When new LAALPs are found or old ones are withdrawn compared to its
old copy, and they are also configured on RBn, it triggers RBn to
perform the "Member RBridges Auto-Discovery" procedure described in
Section 4.1.
9.2. PN-RBv APPsub-TLV
The PN-RBv APPsub-TLV is used by a Designated RBridge of a Virtual
RBridge (vDRB) to dictate the pseudo-nickname for the LAALPs served
by the RBv. It is defined as a sub-TLV of TRILL GENINFO TLV [RFC7357]
and is distributed in E-L1FS FS-LSP [rfc7180bis]. It has the
following format:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = PN-RBv | (2 bytes)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length | (2 bytes)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RBv's Pseudo-Nickname | (2 bytes)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LAALP ID Size | (1 byte)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+...+-+
| LAALP ID (1) | (variable)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+...+-+
. .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+...+-+
| LAALP ID (n) | (variable)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+...+-+
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o PN-RBv (2 bytes): Defines the type of this sub-TLV, #tbd2.
o Length (2 bytes): 3+n*k bytes, where there are n LAALP IDs, each
of size k bytes. k is found in the LLALP ID Size field below. If
Length is not 3 plus an integer time k, the sub-TLV is corrupt and
MUST be ignored.
o RBv's Pseudo-Nickname (2 bytes): The appointed pseudo-nickname for
the RBv that serves for the LAALPs listed in the following fields.
o LAALP ID Size (1 byte): The size of each of the following LAALP
IDs in this sub-TLV. 8 if the LAALPs listed are MC-LAGs or DRNI
(Section 6.3.2 in [802.1AX]). The value in this field is the k
that appears in the formula for Length above.
o LAALP ID (LAAP ID Size bytes): The ID of the LAALP. See Section9.4.
This sub-TLV may occur multiple times with the same RBv pseudo-
nickname with the meaning that all of the LAALPs listed are
identified by that pseudo-nickname. For example, if there are LAALP
IDs of different length, then the LAALP IDs of each size would have
to be listed in a separate sub-TLV.
Since a PN-RBv APPsub-TLV is distributed as part of the application
link state, using the E-L1FS scope [rfc7180bis], changes in contents
or withdrawal or creation of a PN-RBv APPsub-TLV is accomplished by
the Designated RBridge updating and flooding an E-L1FS PDU.
On receipt of such a sub-TLV, if RBn is not an LAALP related edge
RBridge, it ignores the sub-TLV. Otherwise, if RBn is also a member
RBridge of the RBv identified by the list of LAALPs, it associates
the pseudo-nickname with the ports of these LAALPs and downloads the
association to data plane fast path logic. At the same time, RBn
claims RBv pseudo-nickname across the campus and announces RBv as its
child on the corresponding tree or trees using the Affinity sub-TLV
[RFC7176] [CMT].
9.3. PN-MAC-RI-LAALP Boundary APPsub-TLVs
In this document, two APPsub-TLVs are used as boundary APPsub-TLVs
for edge RBridge to enclose the MAC-RI TLV(s) containing the MAC
address information leant form local port of an LAALP when this
RBridge wants to share the information with other edge RBridges. They
are defined as TRILL APPsub-TLVs [RFC7357]. The PN-MAC-RI-LAALP-INFO-
START APPsub-TLV has the following format:
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Type=PN-MAC-RI-LAALP-INFO-START| (2 byte)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length | (2 byte)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-...+-+-+-+-+-+-+
| LAALP ID | (variable)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-...+-+-+-+-+-+-+
o PN-MAC-RI-LAALP-INFO-START (2 bytes): Defines the type of this
APPsub-TLV, #tbd3.
o Length (2 bytes): the size of the following LAALP ID. 8 if the
LAALP listed is an MAC-LAG or DRNI.
o LAALP ID (variable): The ID of the LAALP (see Section 9.4).
PN-MAC-RI-LAALP-INFO-END APPsub-TLV is defined as follows:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type=PN-MAC-RI-LAALP-INFO-END | (2 byte)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length | (2 byte)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
o PN-MAC-RI-LAALP-INFO-END (2 bytes): Defines the type of this sub-
TLV, #tbd4.
o Length (2 bytes): 0.
This pair of APPsub-TLVs can be carried multiple times in an ESADI
LSP and in multiple ESADI-LSPs. When an LAALP related edge RBridge
(say RBn) wants to share with other edge RBridges the MAC addresses
learned on its local ports of different LAALPs, it uses one or more
pairs of such APPsub-TLVs for each of such LAALPs in its ESADI-LSPs.
Each encloses the MAC-RI TLVs containing the MAC addresses learned
from a specific LAALP. Furthermore, if the LAALP is served by a local
RBv, the value of Topology ID/Nickname field in the relative MAC-RI
TLVs SHOULD be the pseudo-nickname of the RBv rather than one of the
RBn's regular nickname or zero. Then on receipt of such a MAC-RI TLV,
remote RBridges know that the contained MAC addresses are reachable
through the RBv.
On receipt of such boundary APPsub-TLVs, when the edge RBridge is not
an LAALP related one or cannot recognize such sub-TLVs, it ignores
them and continues to parse the enclosed MAC-RI TLVs per [RFC7357].
Otherwise, the recipient parses the boundary APPsub-TLVs. The PN-MAC-
RI-LAALP-INFO-START / PN-MAC-RI-LAALP-INFO-END pair MUST occur within
one TRILL GENINFO TLV. If an END is encountered without any previous
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START in the ESADI-LSP, the END APPsub-TLV is ignored. If, after
encountering a START, the end of the ESADI-LSP is reached without
encountering an END, then the end of the ESADI-LSP is treated as if
it were a PN-MAC-RI-LAALP-INFO-END. The boundary APPsub-TLVs and TLVs
between them are handled as follows:
1) If the edge RBridge is configured with the contained LAALP and the
LAALP is also enabled locally, it treats all the MAC addresses,
contained in the following MC-RI TLVs enclosed by the
corresponding pair of boundary APPsub-TLVs, as if they were
learned from its local port of that LAALP;
2) Else, it ignores these boundary APPsub-TLVs and continues to parse
the following MAC-RI TLVs per [RFC7357] until another pair of
boundary APPsub-TLVs is encountered.
9.4. LAALP IDs
The LAALP ID identifies an AAE RBridge Group in the TRILL campus and
thus MUST be unique across the campus. In all of the APPsub-TLVs
specified above, the length of the LAALP ID can be determined from a
size field. If that length is 8 bytes, the LAALP ID is an MC-LAG or
DRNI identifier as specified in Section 6.3.2 in [802.1AX]. The
meaning and structure of LAALP IDs of other lengths is reserved and
may be specified in future documents.
10. OAM Packets
Attention must be paid when generating OAM packets. To ensure the
response messages can return to the originating member RBridge of an
RBv, pseudo-nickname cannot be used as the ingress nickname in TRILL
OAM messages, except in the response to an OAM message that has that
RBv's pseudo-nickname as egress nickname. For example, assume RB1 is
a member RBridge of RBvi, RB1 cannot use RBvi's pseudo-nickname as
the ingress nickname when originating OAM messages; otherwise the
responses to the messages may be delivered to another member RBridge
of RBvi rather than RB1. But when RB1 responds to the OAM message
with RBvi's pseudo-nickname as egress nickname, it can use that
pseudo-nickname as the ingress nickname in the response message.
Since RBridges cannot use OAM messages for the learning of MAC
addresses (Section 3.2.1 of [RFC7174]), it will not lead to MAC
address flip-flopping at a remote RBridge even though RB1 uses its
regular nicknames as ingress nicknames in its TRILL OAM messages
while uses RBvi's pseudo-nickname in its TRILL Data packets.
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The VLAN membership of all the RBridge ports in an LAALP MUST be the
same. Any inconsistencies in VLAN membership may result in packet
loss or non-shortest paths.
Take Figure 1 for example, suppose RB1 configures VLAN1 and VLAN2 for
the link CE1-RB1, while RB2 only configures VLAN1 for the CE1-RB2
link. Both RB1 and RB2 use the same ingress nickname RBv for all
frames originating from CE1. Hence, a remote RBridge RBx will learn
that CE1's MAC address in VLAN2 is originating from RBv. As a
result, on the returning path, remote RBridge RBx may deliver VLAN2
traffic to RB2. However, RB2 does not have VLAN2 configured on CE1-
RB2 link and hence the frame may be dropped or has to be redirected
to RB1 if RB2 knows RB1 can reach CE1 in VLAN2.
How LAALP implementations maintain consistent VLAN configuration on
the TRILL switch LAALP ports is out of scope for the TRILL protocol.
However, considering the consequences that might cause by the
inconsistency, TRILL switches MUST disable the ports connected to an
LAALP with inconsistent VLAN configuration.
It is important that if any VLAN in an LAALP is being mapped by edge
RBridges to an FGL [RFC7172], that the mapping MUST be same for all
edge RBridge ports in the LAALP. Otherwise, for example, unicast FGL
TRILL Data packets from remote RBridges may get mapped into different
VLANs depending on which edge RBridge receives and egresses them.
It is important that RBridges in an AAE group not be configured to
assert the OE bit if any RBridge in the group does not implement it.
Since, as stated in [RFC7379], the RBridges in an AAE edge group are
expected to be from the same vendor, due to the proprietary nature of
deployed LAALPs, this will normally follow automatically from all of
the RBridge in an AAE edge group supporting or all not supporting OE.
12. Security Considerations
Authenticity for contents transported in IS-IS PDUs is enforced using
regular IS-IS security mechanism [IS-IS] [RFC5310].
For security considerations pertain to extensions transported by
TRILL ESADI, see the Security Considerations section in [RFC7357].
Since currently deployed LAALPs [RFC7379] are proprietary, security
over membership in and internal management of active-active edge
groups is proprietary. If authentication is not used, a rogue RBridge
that insinuates itself into an active-active edge group can disrupt
end station traffic flowing into or out of that group. For example,
if there are N RBridges in the group, it could typically control
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1/Nth of the traffic flowing out of that group and a similar amount
of unicast traffic flowing into that group. For multi-destination
traffic flowing into that group, it could control all that was in a
VLAN for which it was DF and it can exercise substantial control over
the DF election by changing its own System ID.
For general TRILL Security Considerations, see [RFC6325].
13. IANA Considerations
IANA is requested to allocate code points tbd1, tbd2, tbd3 and tbd4
from the range below 255 for the 4 TRILL APPsub-TLVs specified in
Section 9 and add them to the TRILL APPsub-TLV Types registry as
follows:
Type Name Reference
---- -------------------------- ---------------
tbd1 PN-LAALP-Membership [this document]
tbd2 PN-RBv [this document]
tbd3 PN-MAC-RI-LAALP-INFO-START [this document]
tbd4 PN-MAC-RI-LAALP-INFO-END [this document]
14. Acknowledgments
We would like to thank Mingjiang Chen for his contributions to this
document. Additionally, we would like to thank Erik Nordmark, Les
Ginsberg, Ayan Banerjee, Dinesh Dutt, Anoop Ghanwani, Janardhanan
Pathang, Jon Hudson and Fangwei Hu for their good questions and
comments.
15. Contributing Authors
Weiguo Hao
Huawei Technologies
101 Software Avenue,
Nanjing 210012
China
Phone: +86-25-56623144
Email: haoweiguo@huawei.com
H. Zhai, et al [Page 28]